(Schiff Base)Mn(III)-Catalyzed Oxidation of Benzylic Hydrocarbons Using Iodosobenzene as the Oxidant

The oxidation of benzylic C-H bonds constitutes one of the most fundamental transformations in organic synthesis.1 High valent metal oxides such as CrO3 have long been employed as the oxidant in the hydrocarbon oxidation.2 However, stoichiometric use of metallic oxidant could cause a problem associated with either an environmental pollution or difficult separation of the metal reagent from the product. Therefore, as an advanced process, catalytic use of metal complexes in combination with organic oxidant has attracted an attention for the activation of hydrocarbons.3 Previously we have reported that (salen)Mn(III)Cl complex 1 could be used as a catalyst in the benzylic C-H oxidation using NaOCl or PhI=O as an oxidant.4 During the process of this study, we were needed to find the more efficient catalyst. Thus, we decided to investigate other (schiff base)Mn(III) complexes for the catalyst. Described here is the result of our observation on this subject. For the screening of the catalytic activity, we have synthesized several (schiff base)Mn(III) complexes 1-8 according to the literature procedure.5 The catalytic activity of the complexes was examined using ethylbenzene as the model substrate. The result is summarized in Table 1. The reaction was carried out using 4 mol % Mn(III) complexes and 3.0 equiv. of iodosobenzene as the oxidant. The reaction was monitored using gas chromatography using 4’-chloroactophenone as the internal standard.6 Employment of the (salen)Mn(III) complex 1, the catalyst used in the previous report,4 afforded the product (acetophenone) in 39 % yield (53 % conversion). Trial of the other salen-Mn(III) complexes (2, 3) provided slightly lower conversion yields (entries 2, 3). When the complex 4, where 1,2-diaminobenzene-derived ligand was coordinated, was used as the catalyst, slightly higher conversion and product yields waere observed. The complex 5, where Cl-substitute was introduced in the ligand, gave no better activity over the